First metrics functional test: Dismiss All Notifcations

Uses the new metrics reader class to inspect the logs.

Test: mmma -j20 platform_testing/tests/functional/notificationtests && adb install -r -g $OUT/data/app/NotificationFunctionalTests/NotificationFunctionalTests.apk && adb shell am instrument -w -e iterations 1 -e class com.android.notification.functional.NotificationInteractionTests#testDismissAll com.android.notification.functional/android.support.test.runner.AndroidJUnitRunner && runtest --path platform_testing/libraries/metrics-helper/tests
Change-Id: I7594461393779e1484f720a4bdc18fdd3d11ea99